Since voltage comparators generally offset the input voltage, it is necessary to correct or eliminate this offset voltage in order to prevent an erroneous operation of the circuit.
FIG. 1 illustrates an example of a conventional comparator utilizing a single operational amplifier (to be referred to hereinafter as "Op Amp" or "OA", in short) having the aforementioned offset voltage cancel function. In the example shown, the offset voltage is corrected (i.e., eliminated) by providing a capacitor Cl between the negative input Op Amp (OA) 1 and the input select switch S1, and further by providing a feedback circuit using a switch S2 placed between the negative input and output (Vout) of OA 1.
FIG. 2 illustrates a second conventional voltage comparator circuit, wherein the comparator of FIG. 1 is now structured as a voltage comparator having two OAs operating in opposite phases to secure a rapid operation. The voltage comparator shown in FIG. 2 uses two Op Amps OA1 and OA2 operating in opposite phases, and a third one, i.e., output OA 3, connected to the two inputs OAs such that the respective outputs of the two OAs become the inputs of OA 3. A switch S1 is connected to the negative input of OA 1, while the positive input of OA 2 is coupled to capacitor C1. Switch S1 switches between input voltage Vi and a reference voltage Vref, alternatively supplying these voltages to the negative input and positive input of the OAs in question. Additionally, a feedback circuit consisting of switch S2 placed between the negative input and the positive output (Vout) of OA 1 is also provided. Moreover, a voltage Vm is supplied to the positive input of OA 1 and to the negative input of OA 2.
The example shown in FIG. 2 has a shortcoming of being unable to cancel the offset voltage of OA 2, due to the circuit configuration, since feedback is provided only to one side of OA 1. In order to avoid this limitation, it is necessary to provide OAs such that the offset between two OAs 1 and 2 becomes uniform. However, it is difficult and unrealistic to obtain a totally uniform pair of OAs using standard manufacturing semiconductor processes. Accordingly, a limitation exists in suppressing the offset voltage of the Op Amps when they depend on the manufacturing process used.